In the past, aluminum alloys have been used for stationary type information devices such as desktop computers and servers, and glass substrates have been employed for portable type information devices such as notebook computers and mobile computers as recording medium substrates. Surface-smoothness of the substrate after polishing tends to be insufficient since not only aluminum alloys are deformable, but also their hardness is insufficient. Further, there was another problem such that a magnetic film was easy to be peeled off the substrate when a recording head was mechanically brought into contact with a magnetic disk. Thus, glass substrates exhibiting reduced deformation, excellent surface-smoothness and high mechanical strength are expected to be utilized from now on for stationary type information devices as well as portable type information devices, and also for other home appliances such as TV and so forth.
Glass substrates to be largely used in this way are subjected to a chemical strengthening treatment which has been utilized in the past in order to improve a mechanical strength so as to bear impact during use of the glass substrates installed in a magnetic disk drive. This chemical strengthening treatment is one by which glass substrates are immersed in a chemical strengthening solution stored in a chemical strengthening treatment tank, and alkali metal ions on the glass substrate surface are substituted by alkali metal ions each having a larger ion diameter than that of the foregoing alkali metal ion to generate compression strain, whereby mechanical strength is to be improved.
Further, higher recording density of a magnetic disk has been strongly demanded. In response to the foregoing, flying height of a recording head is also desired as low as possible. For this reason, higher flatness of the substrate and smaller surface roughness are further demanded. Therefore, there appears an important item to be solved such that not only polishing is conducted with a high degree of accuracy, but also protrusions caused by foreign matter attached onto the surface of the polished glass substrate are not to be generated.
In order to avoid the foreign matter attached onto the surface of the glass substrate, the following methods are cited, for example. (1) Sodium nitrate and potassium nitrate as the chemical strengthening salt are dissolved in ultrapure water, and particles (foreign matter) are removed with a filter for liquid to prepare a chemical strengthening solution (refer to Patent Document 1). Among those, removal of particles of iron including iron oxide and SUS. (2) A holding means to hold at least one of a wall surface and a glass substrate in a chemical strengthening tank is composed of a stainless alloy exhibiting corrosion resistance in the high heating temperature range of a chemical strengthening solution (refer to Patent Document 2).    (Patent Document 1) Japanese Patent O.P.I. Publication No. 2000-203888 (Page 8)    (Patent Document 2) Japanese Patent O.P.I. Publication No. 10-198954